As image forming apparatuses such as printers, facsimiles and copying machines, inkjet recording apparatuses have been known. The inkjet recording apparatus performs the recording by ejecting the ink from a recording head onto a recording medium such as paper, and has advantages in that a making image process is simple and thus the apparatus is easily simplified, the apparatus saves energy and is environmentally friendly because it has no heating process, high-definition images can be recorded at high speed, running cost is inexpensive, noise is low and it is easy to record color images using a multicolor ink.
As such an ink used for the inkjet recording, aqueous inks in which water is a major ingredient, and coloring agents, wetting agents such as glycerin, permeating agents which control permeability into the recording paper, surfactants, and other additives are contained therein have been generally used at home and offices.
Generally, the inkjet recording using the aqueous ink is fixed by permeating the liquid ink into the recording medium. Thus, the paper made specifically for the inkjet printer having enhanced absorbability, fixation of coloring components on the paper surface and protection functions of the coloring components has been developed. However, the paper made specifically for the inkjet printer is expensive because multiple application steps are required after paper making, and is inferior to the plain paper in recycle property because many processing chemicals are used. Thus, it is desired to obtain the sufficient image quality on the plain paper.
Such plain paper is inferior in ink absorbability compared with the paper made specifically for the inkjet printer, and does not assist ink performances as the specifically made paper does, and thus has the following problems. That is, (1) occurrence of feathering, (2) occurrence of bleeding, (3) reduction of density, (4) reduction of color forming property, (5) reduction of water resistance, (6) reduction of light resistance, (7) reduction of gas resistance, (8) reduction of fixing property, (9) show through of ink, and the like. It is an important problem for the inkjet recording on the plain paper to solve the above problems.
Recently, pigments have been frequently used for the inkjet ink because dispersibility has been improved and particle diameters have been minimized. As the improvement of the dispersibility of the pigment, not only pigment is dispersed using surfactants and water soluble resins as conventional cases, but also hydrophilicity is imparted to the pigment surface by surface modification such as oxidation, sulfonation or graft polymerization. Thus, self dispersion stability of the pigment is enhanced. By the use of such pigments, the above (5), (6) and (7) can be improved, but the pigment is inferior to dyes in density and color forming property, and further inferior to dye inks in terms of reliability of jet stability, long term storage stability and redispersibility. Thus, when the pigment is used as the coloring agent, it is the problem to enhance the density, the color forming property and the reliability.
In order to address these problems, for example, the inks for the inkjet recording using colored polymer particles, particularly emulsion of polyester or vinyl based polymer particles have been often reported (see Patent Document 1). These include the ink containing a coloring agent including resin dispersion body in which the coloring agent has been included in a water insoluble dispersible resin. When a color organic pigment is used as the coloring agent, it has been known that this pigment is more excellent in image density and color reproducibility on the plain paper in an ink formulation known publicly and conventionally than the pigment ink using an aqueous dispersant.
Conventionally, in order to reduce the feathering and the bleeding, enhance the printing density and the color forming property and suppress the show through, the technology which controls the permeability of the ink into the paper has been studied. Among the inkjet inks in the market, there is a superpermeable ink where the permeability into the paper has been enhanced by adjusting a surface tension of the ink to less than 35 mN/m. Such an ink has highly effective for reducing the bleeding and is good in drying property in printing on the plain paper, but has shortcomings in that the feathering easily occurs, the printing density is low and letter quality is easily deteriorated. Meanwhile, a slowly permeable ink where the ink is retained on a printing surface layer by adjusting the surface tension of the ink to more than 35 mN/m to slow the permeation into the paper has been placed on the market. This is highly effective for the reduction of the feathering, the enhancement of the printing density, the enhancement of the color forming property and the reduction of the show through. However, in this slowly permeable ink, as a result of reducing the permeability into the paper, the drying property on the plain paper after printing is remarkably poor, leading to deterioration of the fixing property and the bleeding between colors when multiple colors are used.
Thus, the technique to suppress the bleeding between the colors and assure the image quality using an ink set by combining the above slowly permeable ink and the above superpermeable ink has been developed. However, when the printing on both sides is performed using this ink set, a time to wait for drying the ink after the printing is required, leading to the deterioration of productivity in the printing on both sides. Various inkjet recording apparatuses having a device to facilitate the ink dry by heating the paper surface with a heater before and after the printing for enhancing the ink drying property have been proposed and placed on the market (see Patent Documents 2, 3, 4, 5 and 6).
These proposals are problematic in that the advantages of the inkjet mode are lost, i.e., because of having a heating means, the apparatus grows in size and is complicated, and the energy for heating is wasted.
In addition to the above, for the inkjet inks, methods combining both the enhanced reliability and the enhanced image quality have been studied. Many inkjet inks are designed to minimize the increase of viscosity in order to prevent nozzle clogging. For example, in Patent Document 7, it has been described that by making the viscosity change within 10 times and making the particle diameter change within 3 times when the ink is concentrated to 2 times, it is possible to prevent aggregation of the pigments from suppressing spread of the ink to prevent white spotting. However, by this ink, it is difficult to form high quality images on the plain paper. Also in Patent Document 8, the ink where a residual component after evaporating a volatile component in the ink is a liquid and its viscosity is within 10 times of the initial viscosity has been proposed. However, this ink is the dye ink, and although the reliability is high, the image quality on the plain paper is inferior. In Patent Document 9, the ink where the ink viscosity is within 600 times or less before the evaporation when water is evaporated under the environment at 60° C. has been disclosed. However, this ink is also the dye ink, and the reliability of the ink and durability of the image quality are balanced by adding a water soluble polymer, but the water resistance remains problematic. In Patent Document 10, it has been proposed that the ink whose viscosity is 5 to 15 mPa·s is required for assuring the high image quality, and that a particular compound as a viscosity adjuster for adjusting an initial evaporation rate to assure the reliability and adjusting the viscosity is added. However, in this proposal, there is no description for the stability of the pigment particle diameter used, there is the reliability after leaving stand for 24 hours, but the ink is inferior in reliability depending on the constitution of the ejecting head and the size of the nozzle when left stand for a longer time.
This way, it is necessary to use the ink with high viscosity for assuring the printing quality with the high speed and high quality, but in the ink with high viscosity, it is difficult to assure the reliability.
For the purpose of improving the image quality, for example, in Patent Documents 11 and 12, the technique in which a water insoluble resin is added to the pigment ink has been proposed. In Patent Document 13, the technique in which the high quality image is obtained on the plain paper by the ink where a ratio of the pigment to a resin emulsion is in the range of 1:0.1 to 1:1 and an average particle diameter of the coloring component is 0.3 to 1.2 μm has been proposed. However, in such an ink where the resin emulsion has been added, the bleeding is suppressed and the feathering can be suppressed, but the image density is insufficient and the reliability as the inkjet ink is not sufficient. In Patent Document 14, the ink where a lowest film-forming temperature of the ink in which the water insoluble resin has been added is 40° C. or above has been disclosed. In Patent Document 15, the ink where the particle diameter of the added emulsion is 50 nm or less has been disclosed. However, there is a room to improve the reliability, and the resulting image quality is not sufficient. In Patent Document 16, it has been disclosed that printing unevenness is improved by the ink containing the pigment, saccharides or a derivative thereof, a polyol compound having 5 or more hydroxyl groups and a resin emulsion. However, in this ink, the permeability into the plain paper is low, and there are problems such as occurrences of feathering and bleeding, deterioration of the fixing property and slow drying. In Patent Document 17, the ink where a solid content composed of a water-dispersible resin and a self dispersible pigment is 1.0% to 16% by mass has been disclosed. It has been set forth that the image quality is improved on the plain paper, and the water resistance is further improved compared with the dye ink, but an image retention performance for a marker pen frequently used on the plain paper is not sufficient.
When the pigment dispersion body and the resin emulsion are added to the ink, the solid content is increased to increase the viscosity. For example, in Patent Document 18, the formulation where the increase of the viscosity is low even when the solid content is increased by adding the resin which forms a micelle association body has been proposed. However, in this proposed ink, the permeability into the plain paper is short, and the recorded image quality is problematic as is the case with Patent Document 16.
The permeability for the paper is described by capillary absorption represented by Lucas-Washburn, formula, and the higher the viscosity is, the sufficient permeability is not obtained unless the surface tension and a contact angle of the paper and the ink are made low. For example, in Patent Documents 19 and 20, when the permeability is enhanced by using polyhydric alcohol alkyl ether for the ink with high solid content concentration, the solid content is increased to increase the viscosity, and the sufficient permeability is not obtained to reduce the image quality.
It has been known that a silicone based surfactant and a fluorine based surfactant in a trace amount enhance the permeability. For example, in Patent Documents 21 and 22, the inkjet ink in which the fluorine based surfactant has been added has been proposed. Also in Patent Documents 23 and 24, the ink containing the pigment dispersion body and the fluorine based surfactant has been proposed. In Patent Document 25, the ink with a viscosity of 5 mPa·s containing the fluorine based surfactant and a polymer emulsion containing a water insoluble or water hardly soluble color material in polymer fine particles has been proposed. However, in these proposals, preferable hue is not difficult to be contained at high pigment concentration, and the self dispersion pigment can not be said to be sufficiently water resistant and have the sufficient fixing property.
Therefore, for the problems in the inkjet recording on the plain paper, various improvements have been attempted, e.g., the water resistance, the light resistance and the gas resistance are enhanced by using the water-dispersible coloring agent, or the image density is enhanced, the ink show through is reduced and the image quality is improved to suppress the feathering and the bleeding by using the ink having the high permeability and the high viscosity. However, it is an actual circumstance that no ink having sufficiently satisfied performance has been provided yet and the problems remain in color forming property.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 2000-191972    [Patent Document 2] JP-A No. 55-69464    [Patent Document 3] JP-A No. 55-84670    [Patent Document 4] JP-A No. 58-72460    [Patent Document 5] JP-A No. 62-130863    [Patent Document 6] JP-A No. 62-130864    [Patent Document 7] JP-A No. 2002-337449    [Patent Document 8] JP-A No. 2000-95983    [Patent Document 9] JP-A No. 09-111166    [Patent Document 10] JP-A No. 2001-262025    [Patent Document 11]JP-A No. 55-157668    [Patent Document 12] Japanese Patent Publication Laid-Open (JP-B) No. 62-1426    [Patent Document 13] JP-A No. 04-332774    [Patent Document 14] Japanese Patent (JP-B) No. 2867491    [Patent Document 15] JP-A No. 04-18462    [Patent Document 16] JP-B No. 3088588    [Patent Document 17] JP-A No. 2004-35718    [Patent Document 18] JP-A No. 2004-99800    [Patent Document 19] JP-A No. 2004-155867    [Patent Document 20] JP-A No. 2004-203903    [Patent Document 21] JP-B No. 2675001    [Patent Document 22] JP-B No. 2667401    [Patent Document 23] JP-A No. 04-211478    [Patent Document 24] JP-A No. 2003-277658    [Patent Document 25] JP-A No. 2003-226827